SHORT-TERM VARIATION IN THE BODY-WEIGHT OF OYSTERCATCHERS HAEMATOPUS-OSTRALEGUS - EFFECT OF AVAILABLE FEEDING TIME BY DAY AND NIGHT, TEMPERATURE AND WIND FORCE

The available feeding time of coastal Oystercatchers varies from day t o day due to the effect of wind direction and wind force on the water level. If the birds are not able to feed at all during a day, they los e 30 g, or 6% of their body weight. The body weight increases with the duration of the available feeding time, irrespective whether it is da y or night. Oystercatchers continue to feed at night, at least in autu mn and winter. Although wind force and wind direction affect the daily duration of the available feeding time, this variation fades away if calculated over a number of days, and therefore does not affect the bi rds in the long-term. Does the body weight increase, or decrease, with the higher costs of living associated with low temperatures and stron g winds? A decrease in body weight with increased cost of living would suggest. that the birds are not able to find the extra food required to compensate for the higher maintenance level ('undercompensation'). An increase in body weight, on the other hand, would suggest that the birds in these difficult circumstances eat even more than needed in or der to increase their body reserves in cases still worse conditions ar rive ('overcompensation'). Unfortunately, the field data are confusing . The weak increase in body weight at low temperatures suggests an ove rcompensation, but the observed clear decrease in body weight with str ong winds suggests an undercompensation. However, the increase of body weight with lower temperature is not large and is possibly due to int ervening variables, so it is not clear whether this was an actual over compensation. The negative effect of wind force on body weight is pres umably caused by undercompensation in combination with a decrease in t he feeding success.